Lateral Soil Pressure against 2 storey basement wall

[ajk1]

In 1956±, what was the practice for calculating the lateral earth pressure against a basement wall?


From archival calculation notes on microfilm, I see the calculations were carried out based on 100 pounds per cubic soil weight, a lateral earth pressure coefficient of 0.30, no live load surcharge (although the wall is tight adjacent a wide sidewalk of a main street) and by the working stress design method and a rebar working stress of 20,000 psi. Today we would design for 130 pcf soil and probably a lateral pressure coefficient of 0.50, although I am not sure exactly what would be used for clay, and probably a live load surcharge of 200 psf, but at least 100 psf, but I cannot get the wall to figure for this by a wide margin, even if I do it by LSD. The wall was poured against wood lagging and steel soldier piles at 8 foot centres. There is a relatively wide (0.90 mm) horizontal crack at mid-height of several bays of lower basement wall, but not in all bays There are 2 below grade levels of parking. The wall was reinforced with #6 @ 21" verical inside face rebar, and also had heavier out side face vertical rebar at the negative moment region.

I am interested in all comments, but particularly if you had experience in the Toronto area in about that era.

To reiterate, the question is what was the design practice with respect to soil weight and lateral pressure coefficient in about 1956 for basement walls?

 

[XR250]

To reiterate, the question is what was the design practice with respect to soil weight and lateral pressure coefficient in about 1956 for basement walls?


From archival calculation notes on microfilm, I see the calculations were carried out based on 100 pounds per cubic soil weight, a lateral earth pressure coefficient of 0.30, no live load surcharge (although the wall is tight adjacent a wide sidewalk of a main street) and by the working stress design method and a rebar working stress of 20,000 psi.

Seems like you answered your own question :>

 

[ajk1]

No not at all. That is what an engineer who was noted for getting into trouble, designed it for... I know of one of his structures that had a particla collapse.

My question was not what was this wall designed for...to that I know the answer...but my question was what was the "practice" at that time. It is very hard to believe that was the general practice at the time or we would see a lot of basement walls with wide horizontal cracks. My experience starts in 1964, which is not that far away from 1956 I suppose, but unfortunately I cannot recall for sure what we used to do at that time, though I think we took the soil as 120 pcf and the lateral pressure coefficient as 0.40. Some firms at the time may have taken the lateral coefficient as 0.35, but I doubt that they combined it with zero live load surcharge...I am hoping BARetired sees this and can comment...

 

[msquared48]

Section 2309 of the 1955 UBC said 30 psf/ft for "Retaining Walls". This would be for a yielding wall, free to move at the top. Probably not what he had due to floor diahragms.

I could find nothing for "basement walls", or non-yielding walls, that should, by modern codes, be designed to 50 psf/ft. A two story wall should really be designed as a two span beam section, continuous at the intermediate support.

Mike McCann, PE, SE (WA)

 

[oldestguy]

I think you should know more of the backfill type. That 100 may be right for the material then and even now, such as a uniform sand. However, the .30 for active pressure coefficient was common. Since about 1970 or so we got more into pressure at rest with the 0.50 coef. I started about 1956 with investigations and reports.

Stating your reason may help some also. Water conditions along the line may have changed. How about surcharge loads not planned for? Any Frost?

 

[BAretired]

Engineers were certainly aware of active, at-rest and passive pressure in 1956. A two story basement wall is close to the at-rest condition because of the resistance to movement at each of the floors but it is likely that many engineers of that day used tables such as those found in the CRSI Design Handbook which were closer to the active case.

BA

 

[ajk1]

To msquared - it was of course designed as a 2-span beam section, but using approximate coefficients (such as WL/8) to determine the negative moment and other approximate coefficient for the positive moment. It could of course have been designed by moment distribution, but it wasn't. I will check by how much the moment differs from the approximate coefficient method versus an elastic analysis, but I suspect that is not the main issue, as experienced designers could get quite close to the elastic analysis answer by using approximate coefficients.

To oldestguy: the native material is definitely not a uniform sand. Although I don't have boreholes for the site, in that part of the city the soil is not generally a sand material. From boreholes we made 2 weeks ago thru the lowest slab-on-grade down 10 feet below the slab, it was found to be a stiff clay. Anyway, it is interesting that your recollection that 0.30 for K was common then for basement walls. I know my CRSI Handbook from 1964 used that for cantilever retaining walls but I always thought that was because the top of the cantielever wall could move. Unfortunately there are no basement wall designs in the CRSI Handbooks, as far as I know. But your confirmation that 0.30 was also commonly used even for rigid basement walls is useful information. Thanks. Frost is not likely an issue because the top of the lower storey wall is more than 10 feet below grade, and the garage is heated.

To BAretired: Yes that is in line with my thinking. But it seems to using such a low soil weight combined with such a low pressure coefficient, and no live load surcharge may have been pushing the envelope...even back then...what do you think?

(as for our firm's policy when I started in 1964, to the best of my recollection we used 0.40 earth pressure coefficient [a conservative "active" pressure coefficient], and 120 pcf soil weight and working strength design. For a wall adjacent to a main city sidewalk, I think we may have also had a 200 psf surcharge, although I am not certain when we applied the 200 psf surcharge). When we adopted USD in about 1968, we recognized that we should be using the "at rest" coefficient and adopted 0.45. Later geotechnical consultants started saying in their reports to use about 130 pounds per cubic foot for the soil weight, which of course we adopted)

 

[BAretired]

To assume active pressure for vertically spanned basement walls is definitely "pushing the envelope" and I am not recommending it but I suspect it was used by a number of engineers at that time.

For stiff clay soil, the lateral pressure is not quite as easily determined as for granular material but K_o can be higher than 0.5 for some clay soils. Using a soil weight of 120pcf, the equivalent fluid pressure is about the same as water pressure.

BA

 

[ajk1]

Thanks